The present invention relates to a light-emitting element such as a light-emitting diode, to an array of such light-emitting elements, and to a method of fabricating such an element or array.
Light-emitting diodes or LEDs are widely used as display devices, because they emit bright light and can be driven at a low voltage by simple circuitry. For similar reasons, LED arrays are used as light sources in electrophotographic printers. A conventional LED array is fabricated by selectively diffusing a p-type impurity such as zinc into an n-type gallium--arsenide substrate to form an array of pn junctions. Each pn junction functions as a LED. Current is fed to the LEDs through an array of aluminum electrodes formed on the emitting surface of the array. The current returns through a common gold-germanium electrode formed on the opposite surface of the array.
In conventional LED arrays, the light-emitting diffusion area of each LED is partly covered by the aluminum electrode, but this arrangement can create certain problems. If too much of the diffusion area is covered, then too much of the emitted light is blocked by the aluminum electrode. If the covered area is too small, however, then the contact resistance between the diffusion region and electrode becomes too high. In either case, the light-emitting efficiency of the LED is reduced.
The problem of reduced light-emitting efficiency becomes particularly serious when the diffusion region has a small sheet resistance, because then most of the light emission takes place directly below the aluminum electrode. The problem also becomes serious in high-density arrays. In an array with a linear density of one thousand two hundred dots per inch (1200 DPI), for example, the small contact area between the LEDs and their electrodes is subject to relatively large fabrication variations, which can create large differences in light-emitting characteristics among the LEDs in the array. In the extreme case, the contact area can become too small for a LED to be driven.